1. Field of the Invention
The present invention relates to a multi-layered heat resistant metal tube having excellent anti-coking characteristics and a method for manufacturing thereof. The multi-layered heat resistant metal tube is suitable for those tubes which are used in a high temperature and in a high carbonization potential environment as components of apparatus.
2. Description of the Related Art
High resistance to coking and carbonization, in addition to heat resistance, is required to some parts such as heat radiant tube of carburization-hardening furnaces, cracking tubes of thermal decomposition furnaces, particularly, ethylene cracking furnace tube or oil refinery cracking tube or all the other petrochemical thermal cracking furnace tube applications. Coking is a phenomenon of deposition and accumulation of carbon generated by thermal decomposition of hydrocarbons onto the surfaces of the metal tubes. This causes trouble of decrease in cross sectional area of furnace tubes, which may finally lead to blocking of the furnace tubes. Carbonization of metal is a carburization phenomenon of intrusion of carbon through the surface of the metal and diffusion of carbon into the inner part of the metal. This may either cause the coking or directly be processed by intrusion of carbon from the atmosphere. In any way, corrosion of the tubes due to decreased corrosion resistance caused by the carburization, decrease in the area of inner diameter of the tubes due to coke deposit, and possible embrittlement subsequently caused is fatal to the furnace tube units.
There exists a single layer solid tube of Crxe2x80x94Ni base heat resistant alloy in application for the above listed use. A group of the conventionally known materials are composed of 40-50 wt % Crxe2x80x94Ni alloys disclosed in Unexamined Japanese Patent Publication (kokai) Nos. 05-93240, 07-113139, 07-258782 and 07-258783. The alloys were designed to contain, in order to provide high temperature strength, 0.1-0.5 wt % of C and not more than 0.2 wt % of N, and additionally contain, as the components to enhance the high temperature strength, at least one element of Al, Nb, Ti, Zr or W. Another group is represented by an alloy steel disclosed in Unexamined Japanese Patent Publication (kokai) No. 05-1344. This alloy steel has alloy compositions containing 0.05-0.3 wt % of C and 0.1-0.6 wt % of N with the purpose of providing high temperature strength, not more than 5.0 wt % of Si as the component of giving resistance to carburization, and not more than 0.4 wt % of Mn and 0.001-0.02 wt % of Mg for improvement of the ductility.
These alloy compositions are, on one hand, effective for the purpose of providing high temperature strength and improving ductility, and on the other hand, could be rather harmful in anti-coking and/or anti-carburizing characteristics, and thus, the conventional materials are not satisfactory from these points of view.
Then dual layered cast tube was developed so that base layer might function for high temperature resistance while the surface layer would work for anti-coking or anti-carburizing. xe2x80x9cInsert castingxe2x80x9d is a typical method for producing the double layered tubes. Unexamined Japanese Patent Publication (kokai) No. 60-170564 discloses a technology to produce bent tubes by using a previously heated insert in a shape of a bent tube and a sand mold as the outer mold and casting molten metal therebetween to obtain a cast product in which the bent tube is inserted. The drawback of this technology is that the surface of the insert which contacts with the molten metal melts and contaminates the molten metal. Further, in case where the cast product is thin, distribution of the molten metal will be insufficient, and defects such as incomplete surface fusion and blow holes often occur.
Another method for producing double layered tubes is centrifugal casting. Unexamined Japanese Patent Publication (kokai) Nos. 05-93238 and 05-93249 propose sequential casting which is composed of the first charging molten metal of high nickel Fexe2x80x94Nixe2x80x94Cr heat resistant steel in a centrifugal casting machine to case the other layer, and then charging a molten metal of Crxe2x80x94Ni alloy to cast and form the inner layer. In order to operate a centrifugal casting machine with high productivity, however, it is forced to charge the molten metal for inner layer before complete solidification of the outer layer. It has been known that, as far as coking is concerned, Fe acts as a catalyst for coke depositing, and therefore, is harmful. In case of combining the above alloy compositions, it is inevitable that Fe contained in the outer tube material diffuses into the inner layer material to reach the surface of the inner layer, and thus, it is not possible to produce multi-layered tube having excellent anti-coking characteristics. On the other hand, casting the inner layer after solidification of the outer layer results in cracking due to thermal expansion-contraction during solidification, and thus, it is quite difficult to produce desired tubes with practical yield.
Possible further way of producing double layered tubes is hot extrusion to form cladded tubes. Unexamined Japanese Patent Publication (kokai) No. 07-150556 proposes tube forming by hot rolling of a blank prepared by inserting a hollow billet of alloy having a suitable alloy composition into a hollow billet of a Nixe2x80x94Fexe2x80x94Cr heat resistant alloy. At present, however, the costs for production is so high that this kind of cladded tubes have not practical use in this field.
xe2x80x9cInchromizingxe2x80x9d method is a technology to form a high chromium layer on the surface of a heat resistant metal tube. Thickness of the chromium rich layer formed by this technology is, however, 30 xcexcm to 50 xcexcm at highest, and thus, there is limitation in application to the parts of apparatus from which surface layer as a consumable will be lost by oxidation or carbonization.
Ethylene cracking reactor furnace (or ETHYLENE TUBE) produces ethylene by cracking naphtha at the toughest conditions where high temperature heat resisting and anti-coking material were critically demanded. Naphtha, as the feed steam mixture, passes through the ETHYLENE TUBE of the radiant section where thermal cracking takes place. The heat of conversion is provided by burners on the side wall or in the bottom of the radiant section, called fire box. During the very short residence time in the radiant coil (tenth of a second) at around 1000xc2x0 C., the hydrocarbons including naphtha are cracked to ethylene, butadiene, butanes and aromatics. Since the tube material temperature in the fire box can often exceed 1100xc2x0 C. (2021xc2x0 F.), centrifugal cast high temperature, creep resistant alloys such as HK-40, HP-40 or HP Mod. are in use. While these conventional materials are good for creep rapture at 1100xc2x0 C., precipitation and depositing of carbon and/or embrittlement of ETHYLENE TUBE accompanied with carbonization are inevitable. Built-up carbon coke needs a cyclical removal of coke which is accompanied with interrupting the operation (anti-coking). Embrittlement by carbonizing in a serious case needs replacement of the whole tube unit. Because of gas stream inside the tube being of hydrocarbon at high temperature the gas atmosphere will be origin for building carbon deposition on the surface of the tube metal. This will reduce run-length and also leads to subsequent diffusion into the tube material. The diffusion process or carbonization will cause many detrimental effects in the physical properties of the tube. The ductility, toughness, rupture time and melting will deduce the original grade progressively as the carbonization process goes on. This can eventually lead to prematured failure of the tubes by a set of attacks of thermal shock, stress rupture, thermal fatigue, or carbonization-induced cracking. In fact carbonization is the major cause of ethylene furnace failure in industry wide.
At the 11th Conference of Ethylene Makers held in Houston in March of 1999 there was a presentation for improvement of the ethylene tubes to give double layer coating of a Crxe2x80x94Si alloy and Sixe2x80x94Al alloy on the substrate tube made of heat resistant alloy. This technology is effective to improve anti-coking characteristics. However, the coating needs huge investment for a metal powder, a ceramic powder and a polymer, and further, requires complicated steps to carry out the coating, i.e., a chemical treatment step for adhesion to form three layers which are an adhesion layer, diffusion layer and hard non-reactive layer, and inactivation of the surface layer. Thus, costs for preparing the double coated ethylene tubes are very high.
An object of the present invention is to provide a heat resistant metal tube which can be used under the conditions where carbon deposit and carburization easily occur at a high temperature, typically, in ethylene tubes of naphtha crackers, with improved performance of excellent anti-coking characteristics and anti-carburizing characteristics in the form of a multi-layered metal tube.
The multi-layered heat resistant metal tube having excellent anti-coking characteristics according to the present invention is made by forming a weld-mounted overlaid layer of a Crxe2x80x94Ni alloy by building-up welding layer on the inner surface and/or the outer surface of a substrate tube of heat resistant metal. In the tube, the alloy of the weld-mounted layer overlaid by building-up welding comprises 35% by weight or more of Cr and satisfies the relationship: Ni(w %)xe2x89xa70.5Cr(wt %).